It is often necessary, in uneven terrain, to form the subgrade of a highway or of a railroad track by the coast or at the foot of a piece of ground with a steep gradient, sometimes even a vertical cliff, for example at the exit of a tunnel.
Such a structure is intended essentially to protect the highway against landslides or avalanches. It must therefore consist, on the uphill side, of a solid wall which is joined to a covering enabling the landslides or avalanches to be held back or alternatively to pass over the top of the highway. On the downhill side, on the other hand, the covering generally bears on a wall pierced with orifices, often a series of pillars, so as to permit natural lighting of the highway and not to extend the tunnel purposelessly.
The covering must be able to sustain very substantial impacts caused, for example, by the fall of large blocks, and it is often preferred to give it the shape of a vault. Furthermore, it is preferably covered with an embankment which enables the impacts to be absorbed.
Such a structure must, however, also be able to sustain substantial lateral forces caused by the landslides and the avalanches.
Such structures have been known for some time.
U.S. Pat. No. 3,282,056 for example, discloses such protective tunnels, but these are constructed entirely from corrugated sheet metal panels which must be held in place by ties. Such panels do not have the same strength as a reinforced concrete wall and, in particular, the risks of corrosion do not give the structures constructed in this way sufficient durability. This is why the use of reinforced or prestressed concrete is normally preferred. For example, the journal "Route et Circulation Routiere", No. 6, of June 2, 1967 shows a protective tunnel consisting of an arched concrete wall embedded in the ground on the uphill side and resting on a series of pillars on the downhill side.
Swiss Patent No. 402,919 likewise discloses a tunnel of this type, comprising a covering slab anchored in the rock wall on the uphill side and resting, on the downhill side, on a wall provided with movable panels enabling the downward pressure caused by the passage of an avalanche to be absorbed.
In the past, such structures were constructed using conventional techniques with reinforced concrete, in other words by casting in-situ, using forms and arch centers. These processes take a relatively long time and are relatively expensive.
In mountainous terrain, the season which is favorable for construction is very short. Moreover, workers and equipment are not fully protected against the rock falls or landslides which may occur during construction.